This invention relates to spread-spectrum communications systems, and more particularly to arrangements for determining the frequency or signal timing of such signals.
It is often necessary to detect, acquire, and track the frequency and time of a received direct-sequence spread-spectrum (DS/SS) signal in order to reliably de-spread and demodulate the received signal. In direct-sequence spread-spectrum, the transmitted information bits are multiplied or spread by a high rate pseudorandom (PRN) spreading sequence. In many DS/SS systems, detection and acquisition of received signal timing and frequency is difficult because the signal energy is well below the associated noise level. In code-division multiple access systems in which multiple users access the same frequency bandwidth by use of different spreading codes having advantageous cross-correlation properties, the noise contribution from the codes of the other users adds to the thermal noise to exacerbate the detection and acquisition problem.
Ordinary prior-art approaches to the acquisition problem are based mainly on correlation or matched filtering operations performed on the received signal, variants of which are termed sliding correlation, serial search, and recursion aided sequential estimation. In the presence of a signal, many of these techniques ultimately generate a signal correlation or autocorrelation function, the peak of which should indicate signal timing. However, the noise and other interferences associated with the signal may make determination of the correlation peak difficult. Frequency errors or cumulative phase errors between the information carrier and the local frequency will tend to decrease the magnitude and increase the spread of the peak correlation.
A standard technique for acquisition of signals in the presence of frequency errors or frequency shifts is to use frequency bin searching, either by use of a plurality of correlators operating in parallel, one for each frequency of the received signal, or by searching frequency serially, or both. The use of one correlator for each possible frequency requires a great deal of hardware (or equivalent software), but provides fast acquisition. Serial searching of the frequencies requires only one correlator, but may require significant time to complete a search, especially considering that each attempted correlation may take as long as the recurrence period of the spreading code. Thus, there is a tradeoff between speed and complexity; two correlators could be used in parallel to search using various different frequency hypotheses, thereby presumably cutting the search time in half (by comparison with a single comparator using serial search) by a doubling of the complexity of the equipment.
Correlation or filtering is well known, and can be implemented by many techniques, as for example by Finite Impulse Filters (FIR) or by use of Fast Fourier Transforms (FFT).
Improved techniques for determination of the presence of spread-spectrum signals, or of the frequency and or time of spread-spectrum signals, is desired.
A method according to an aspect of the invention is for detecting the presence of, the frequency or the timing of a received spread-spectrum signal encoded at a known nominal chip rate with a known PRN sequence. The method includes the step of sampling the received signal at an integer multiple, including the multiple unity, of the chip rate, to thereby generate sampled received signals. The sampled received signals are correlated with at least first and second different portions of the PRN sequence with which the signals were originally encoded, to produce correlation outputs when correlation occurs. The samples of the correlation outputs are delayed by an amount equal to the time difference between the different portions of the PRN sequence, to thereby produce a plurality of time-congruent samples. The plurality of time-congruent samples are processed to determine at least the frequency of the received signal.
In one mode of performing the method according to the invention, the step of sampling is performed in a binary fashion, so that the sampled received signals have values of only zero and one. In this method, the step of correlating includes the step of correlating the sampled received signals with the first and second portions includes the step of correlating the sampled received signals with first and second contiguous portions of the PRN sequence. The step of delaying the samples may include the step of delaying the samples by increments of the duration of one of the portions of the PRN sequence.